The present invention relates generally to sealed alkaline batteries of the round cell type (i.e., button cells) which comprise a positive nickel hydroxide electrode and a negative cadmium electrode, each generally having the shape of a pressed powder tablet in a metal wire gauze envelope.
Round cells, especially of the Ni/Cd type, are extensively used for energy supply in electronic entertainment devices to operate miniature motors (e.g., in electric shavers, in hobby tools or in pattern making). Being mass-produced articles, the technology for manufacturing such cells is presently so highly efficient that further improvements in quality hardly seem possible without a simultaneous increase in manufacturing costs. What is more, normal practical requirements are satisfied by the round cells which are presently commercially available.
A state of the art Ni/Cd round cell generally makes use of a positive housing cup and a negative cover, both of which are made from deep-drawn sheet steel and lined with a nickel layer having a thickness of about 5 .mu.m. A sealing ring made of plastic, which also serves to ensure electrical insulation of the cup from the cover, is located in the closure zone which is formed by beading the edge of the cup over the edge of the cover.
The interior of the housing is essentially filled by two paste electrodes which are enclosed as pressed tablets in a fine-meshed metal wire gauze, preferably formed of nickel. The separator, which is made from a nonwoven plastic and impregnated with an alkaline electrolyte, is located between the positive and negative electrode tablets. A uniform contact resistance between the electrodes and the housing is maintained by a corrugated contact spring on the outermost side of the negative electrode.
To prevent gas from accumulating in the cell during cyclic charge/discharge operations, the negative electrode is usually designed to have a larger capacity than the positive electrode (the positive electrode therefore determines the useful capacity of the cell). Part of this negative excess capacity provides a so-called charge reserve consisting of excess cadmium hydroxide, which can be charged and transformed into metallic cadmium. Oxygen evolving from the electrode is taken up by the presently charged portion of the negative electrode, while Cd(OH).sub.2 is formed at the same rate at which new Cd.sub.met. is formed from the charge reserve during further charging. In this manner, the cell is protected from overloading.
The other portion of the negative excess capacity occurs as metallic cadmium in the normally discharged state of the cell (i.e., after exhaustive discharge of the positive electrode) and thus forms a presently available negative discharge reserve. However, concurrently with this discharge, the current which continues to flow in the discharge direction tends to cause electrolytic decomposition of water (forming H.sub.2) at the positive Ni(OH).sub.2 electrode which cannot be further reduced. This leads to the formation of a hydrogen electrode having a potential which is more negative than that of the cadmium electrode, instead of a nickel hydroxide electrode. To prevent the risks which this involves, such as pole reversal of the cell and gas pressure build up, the positive electrode paste is provided with an additive in the form of Cd(OH).sub.2, a so-called antipolar paste (i.e., with a material which is reducible at the same potential as the potential of the negative electrode). This material must be coordinated with the discharge capacity of the negative electrode so that this capacity is exhausted and produces oxygen only if part of the anti-polar additive has transformed into Cd.sub.met.. Under these circumstances, the oxygen can be reduced as in the case of overcharging at the potential for Cd/Cd(OH).sub.2. The cell is therefore also protected from pole reversal.
With a total positive to negative capacity ratio which is once and for all set by an appropriate pretreatment of the electrode, and with the mutually coordinated capacities of the charge reserve portions, the discharge reserve and the antipolar paste, the sealed round cell is able to reliably operate under all usual operating conditions, since the balance of the capacity portions (the so-called cell balance) is not disturbed in a lasting manner.
However, this is not true under extreme loads (e.g., in the case of rapid charging with high currents) since the oxygen consumption at the negative electrode is lower than the rate of formation at the charged positive electrode during the overloaded phase, and the original balance is shifted to the detriment of the charge reserve.
DE-OS No. 35 34 758 discloses that an electro-catalytic active material can be used as an auxiliary electrode with the negative electrode to improve the reduction of oxygen. However, the use of an auxiliary electrode is not successful if complete consumption of the gas takes place, even in the case of overload. Such a requirement is now of great interest for batteries used in practice due to the increasing use of cordless television sets, to permit them to remain operational for several months.